Try to entice him to eat using live foods, adult brine, mysid, or mysis, or ghost shrimp.
If he eats any of that, start enrichment of them before feeding to it.
Once eating again, you can then start it back on frozen foods again.
However, there may be something seriously wrong that is causing the seahorse to not eat.
What you mean is that "all the water parameters that you check" are fine. Most causes of seahorse problems are NOT detected by normal water quality testing that can be used for reef tanks.
A primary cause is bacteria, especially due to the "dirty" water seahorses create, even though your water may look clean.
What size tank do you have? Tankmates? How long have you had it? Water temperature? Do you have a hospital tank? Have you tried a FWD?
It could also be pathogen in origin, especially if it has been exposed to other fish, even in an LFS tank that has had, or is connected to, tank(s) with other fish.
If it has come from off shore (as in net pen, or tank raised), it would be especially prone to having pathogens.
